1. Field of the Invention
The present invention relates to analog amplifier circuits, and in particular, to DC-powered, analog amplifier circuits for receiving DC- or AC-coupled input signals and producing DC- or AC-coupled output signals.
2. Description of the Related Art
Referring to FIG. 1, analog amplifiers, such as audio power amplifiers, are often required to operate from a single power supply. In some cases, this means the AC input signal (VIN) must be AC-coupled to the input of the amplifier circuit since the internal reference for the amplifier circuit is at a DC voltage between DC circuit ground and the power supply potential, e.g., at a value equal to one-half of the applied DC power supply voltage. This DC reference is established by a voltage source, often in the form of a resistive voltage divider (R1 and R2) with a bypass capacitor (CB) for charging to and maintaining the DC reference voltage across the lower resistor. (In this particular circuit, as is often the case for audio power amplifiers, two serially cascaded inverting amplifiers are used in a bridge-tied-load (xe2x80x9cBTLxe2x80x9d) configuration to drive a load, e.g., a speaker, with a differential output signal.)
However, this type of circuit suffers from a problem due to the necessity of having an AC-coupled input. Upon application of DC power (V+) to this circuit, the bypass capacitor begins to charge, as does the input coupling capacitor (CC) which is grounded at the input side by the output impedance of the grounded input signal source. This results in the two capacitors having, at any given points in time during their charging or discharging periods, different voltages across them. In turn, this causes a transient signal to appear across the load. For example, during initial circuit turn-on, the current for charging the input coupling capacitor flows from the output of the first amplifier through its feedback (RF1) and input (RI1,) resistors. The resulting signal at the output of the first amplifier appears at the load in the xe2x80x9cnegativexe2x80x9d portion (VOUTxe2x88x92) of the differential output signal, with the xe2x80x9cpositivexe2x80x9d portion (VOUT+) applied by the second, cascaded inverting amplifier. This initial signal across the load is a turn-on transient which in the case of an audio power amplifier produces a xe2x80x9cclickxe2x80x9d or xe2x80x9cpopxe2x80x9d from the speaker. Similarly, during circuit turn-off, a turn-off transient produced by unequal discharging of the capacitors may produce a xe2x80x9cclickxe2x80x9d or xe2x80x9cpopxe2x80x9d from the speaker as well.
Similar turn-on and turn-off transients occur in single-ended load (xe2x80x9cSELxe2x80x9d) circuits, i.e., those amplifier circuits in which a single-ended output signal is provided to a grounded load (e.g., either VOUTxe2x88x92 or VOUT+ only) rather than a differential output signal to a load isolated from circuit ground. Indeed, whereas in a BTL configuration the outputs may track each other during startup and thereby avoid producing a xe2x80x9cpop,xe2x80x9d an SEL configuration will virtually always produce a xe2x80x9cpopxe2x80x9d unless the output bias reference is at DC ground.
Conventional amplifier circuits have been developed which address this xe2x80x9cpopxe2x80x9d problem in a number of different ways. One approach has been to avoid using single power supply circuits by biasing the amplifier circuit between equal positive and negative power supply voltages with the output driving a grounded load. This allows the input coupling capacitor to be eliminated, thereby eliminating the cause of the turn-on and turn-off transients. However, this requires a second power supply which increases system complexity and costs. Another approach has been to apply the single DC power supply voltage in a gradual manner to initiate the flow of DC bias currents within the amplifiers. However, this results in the amplifier circuit having an indeterminate state of operation during turn-on and turn-off. Further, turn-on and turn-off transients can still occur when power is removed and quickly reinstated as in when a system reset is performed.
Accordingly, it would be desirable to have an analog amplifier which can be operated with a single power supply and reduced DC power related transients.
An amplifier circuit for operating with a selectively variable reference voltage for reducing turn-on and turn-off transients in accordance with the present invention significantly reduces transients in its output signal due to circuit turn-on and turn-off while providing increased flexibility in the selection of values for the reference voltage bypass capacitor and the input signal coupling capacitor. Output signal transients during circuit turn-on and turn-off are more easily predicted due to simpler relationships between circuit variables and the transient output signal waveform, and circuit turn-on and turn-off times can be decreased with less significant increases in output signal transients.
An amplifier circuit for operating with a selectively variable reference voltage for reducing turn-on transients in accordance with one embodiment of the present invention includes an amplifier, a reference generator, and a controller. The amplifier is configured to operate in either a first mode, wherein the amplifier acts as a voltage follower with respect to a first reference voltage, or in a second mode, where the amplifier amplifies an input signal. The amplifier is changed from its first mode to its second mode through the selection of a switch. The switch is controlled by the controller which generates a delayed control signal indicative that the first reference voltage has risen above a fixed, second reference voltage. The delayed control signal is delayed a sufficient amount of time to allow the DC transients in the circuit to dampen before switching the amplifier from its first mode to its second mode.
In accordance with another embodiment of the present invention, the amplifier is further configured to operate in either a turned-on or a shutdown mode of operation, determined by a shutdown signal provided by a controller. A shutdown event causes a first reference voltage to move from its steady state value toward a second value. The controller compares the moving first reference voltage with a fixed second reference voltage to detect that the shutdown event has been triggered. In response to that determination, the controller generates a delayed control signal, and provides that delayed control signal to the amplifier. The controller is configured to generate the delayed control signal a sufficient amount of time after the detection of the shutdown event to allow the DC transients in the circuit to dampen before switching the amplifier from its second mode to its first mode.
These and other features and advantages of the present invention will be understood upon consideration of the following detailed description of the invention and the accompanying drawings.